Horizontal wells are used in conjunction with a variety of treatment technologies to deliver or remove liquids and gases to or from the subsurface. Horizontal wells may be used to enhance other technologies such as Bioventing, Soil Vapor Extraction, Air Sparging, In-well Air Stripping, and Sub-Structure Vapor Depressurization. Horizontal wells are sometimes preferable to vertical wells because they provide greater access to contamination that is moving horizontally, or reach subsurface areas without damaging surface structures. Where land reuse is anticipated, horizontal wells reduce constraints by minimizing the treatment footprint.
Horizontal or directionally drilled wells are positioned horizontally or at an angle. Directional wells can either be double-ended (surface to surface) or single-ended (blind sided). Each type of well has advantages and disadvantages. During well installation, a navigation tool is located close to the drill bit. Slow rotation of the drill head cuts and compacts the soil into the borehole wall. In some cases, cuttings are brought to the surface as is done with vertical drilling. Directional drilling equipment is capable of starting a borehole, steering the drill bit down to a desired horizontal depth, continuing at that depth, and then steering back to the surface. Installation of the well takes place after drilling activities are completed.
Limitations and Concerns
As opposed to vertical wells, horizontal wells may have a greater potential to collapse. For example, special drilling strategies are needed for wet sands because they are difficult to stabilize. Borehole collapse is also more likely in single-ended drilling since the hole is left unprotected between drilling and reaming and between reaming and casing installation. Double-ended holes may be easier to install since reaming tools and well casing can be pulled backward from the opposite opening, and the hole does not have to be left open.
Wells have been difficult to position precisely. It is reported that new guidance technology has limited this concern. Single-ended completion also involves the precise steering of reaming tools required to match the original borehole path.
Currently, the technology is limited by depth. Most installed horizontal wells are less than 50 vertical feet. However there are a few wells that have been completed deeper, the deepest reported at 230 feet. This is because the mechanism for tracking the direction of the well generally decreases with increasing depth of installation, and it may be influenced by site-specific hydrogeologic conditions.
If gas or liquid is going to be injected, lateral movement of contaminants should be monitored.
Each potential site must be assessed for the utility of horizontal wells. Heterogeneous soils with rocks and cobbles may limit the ability to drill.
Directional well technology is used for a range of applications, including groundwater removal, air sparging, free product recovery, in-situ bioremediation, soil vapor extraction, in-situ soil flushing, and leachate containment/collection. It is especially useful when a contaminant plume covers a large area or when surface obstructions are present. The technology is applicable to the complete range of contaminant groups. Recently, this technology has been used to install Sub-Structure Vapor Depressurization system under existing buildings, replacing the conventional method of drilling vertically through floors.
Technology Development Status
Directional drilling equipment is commercially available from a number of vendors. Horizontal drilling was originally developed by industry for oil recovery, utility placement, mining, and construction dewatering. It was modified for the environmental remediation industry in the late 1980s and early 1990s. Over 1,000 horizontal wells have been cataloged as of December 31, 1999. Wells have been completed to over a thousand feet in length.
See http://www.horizontaldrill.com/assets/pdf/DOEHWELLS.pdf for a description of demonstrations at Department of Energy sites.
See http://www.cluin.org/download/toolkit/horiz_o.pdf for a description of remediation technologies using horizontal well drilling methods.